1. Field of the Invention
The present invention relates to a thermionic emission cathode. More particularly, it relates to a thermionic emission cathode bonding a cathode tip made of hexaboride having calcium hexaboride structure and a metallic support with a reaction barrier layer containing colloidal carbon and a preparation thereof.
2. Description of the Prior Art
An alkaline earth metal or rare earth metal hexaboride having calcium hexaboride cubic crystalline structure (hereinafter referring to as hexaboride) usually has excellent physical properties such as small power factor, high melting point, high strength at high temperature, high brightness and long life and accordingly it is useful as a thermionic emission cathode substance. When it is used for a thermionic emission cathode, a reaction of the hexaboride with a metal of the metallic support for supporting is quite severe at an electron emission temperature of about 1500 to 1600.degree. C. In the use of the hexaboride for a thermionic emission cathode, it is necessary to form a reaction barrier layer to prevent the reaction. The reactivity of the hexaboride with carbon is relatively low at high temperature. Therefore, it has been proposed to hold the hexaboride tip by an anisotropic carbon. However, large electric power is required for heating the thermionic emission cathode in this manner. Moreover, it has been difficult to directly hold it on an electron gun of an instrument equipped with a conventional tungsten hair pin type cathode such as an electron microscope shown in FIG. 1. A large power capacity has been required.
It has been proposed to overcome these disadvantages of the conventional thermionic emission cathode in Japanese Unexamined Patent Publication No. 64268/1977 and No. 64269/1977 which propose cathodes placing a reaction barrier layer containing zirconium boride, titanium boride, niobium boride, hafnium boride, chromium boride, zirconium nitride, niobium nitride, vanadium nitride and hafnium nitride, between a hexaboride tip and a metallic support made of tantalum, molybdenum or tungsten. The cathode has the advantage for preventing the reaction of the hexaboride with the high melting point metal such as Ta, Mo and W, however, the bonding property of the reaction barrier layer made of zirconium boride, etc. and the hexaboride is inferior to disconnect the hexaboride cathode tip in the use for a long time.